High reliability lead-free solder alloys for harsh environment electronics applications

ABSTRACT

A SnAgCuSb-based Pb-free solder alloy is disclosed. The disclosed solder alloy is particularly suitable for, but not limited to, producing solder joints, in the form of solder preforms, solder balls, solder powder, or solder paste (a mixture of solder powder and flux), for harsh environment electronics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims the benefit of U.S.patent application Ser. No. 15/147,137 filed May 5, 2016, titled “HIGHRELIABILITY LEAD-FREE SOLDER ALLOYS FOR HARSH ENVIRONMENT ELECTRONICSAPPLICATIONS”, which claims the benefit of U.S. Provisional ApplicationNo. 62/157,302 filed May 5, 2015, titled “High Reliability Lead-FreeSolder Alloys for Harsh Environment Electronics Applications”, each ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to lead-free solder alloycompositions for use in electronics and, in particular, to lead-freesolder preforms, solder powder, solder balls, solder pastes, and solderjoints made of a lead-free solder alloy.

DESCRIPTION OF THE RELATED ART

Since the European Union implemented Restrictions on HazardousSubstances (RoHS) regulations in July 2006, lead (Pb) free solder alloyshave been widely adopted by electronics industries. However, currentPb-free solder alloys are mainly used in non-harsh electronicsenvironments that require a service or operating temperature at or below125° C. For harsh electronics environments, such as for, for example,automotive applications that require operating temperatures at 150° C.or higher, the Pb-free SnAgCu (“SAC”) solders such as Sn3.0Ag0.5Cu(SAC305) and Sn3.8Ag0.7Cu (SAC387) alloys are not reliable enough toreplace the high-Pb, high melting temperature solders.

High-Pb solder alloys are currently one of the most widely used dieattach materials in power semiconductor packages, especially inlarge-sized dies. The microstructures of the high-Pb solders aregenerally quite stable and they do not change much during long-termaging at elevated temperatures. These high-melting, high-Pb alloys,which combine the high-temperature capability and stability with highductility and acceptable thermal/electrical conductivity for mostapplications, are widely used in a range of applications including thepackaging of high power modules.

Harsh environment electronic industries that are currently exempted fromRoHS regulations are actively searching for a suitable replacementsolder. In addition to Pb-free legislation, this search is driven bymore stringent electronics reliability requirements due to theincreasing utilization of electronics in automotive vehicles. Theautomotive industry's trend toward higher power electrical vehiclesrequires that the power modules in the vehicles (e.g., the IGBT module)have a higher efficiency, lighter weight, smaller size, and higherreliability at high operating temperatures. This in turn drives demandfor the use of Pb-free solder alloys with a reliability even higher thanthat of current high-Pb solders. In some semiconductor packagingapplications, it is also intended to reduce the soldering processtemperatures to those of the popular SAC alloys since there are nosubsequent soldering assembly requirements.

In view of the forgoing, it would be desirable to develop a highreliability Pb-free solder alloy to meet the requirements for theseharsh environment electronics applications such as in the automotive anddefense industries.

BRIEF SUMMARY OF EMBODIMENTS

A SnAgCuSb-based Pb-free solder alloy is disclosed. The disclosed solderalloy is particularly suitable for, but not limited to, producing solderjoints, in the form of solder preforms, solder balls, solder powder, orsolder paste (a mixture of solder powder and flux), for harshenvironment electronics. An additive selected from 0.1-2.5 wt. % of Biand/or 0.1-4.5 wt. % of In may be included in the solder alloy.

As used herein, the term “about” in quantitative terms refers to plus orminus 10%. For example, “about 10” would encompass 9-11. Moreover, where“about” is used herein in conjunction with a quantitative term it isunderstood that in addition to the value plus or minus 10%, the exactvalue of the quantitative term is also contemplated and described. Forexample, the term “about 10” expressly contemplates, describes andincludes exactly 10.

Other features and aspects of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, the featuresin accordance with embodiments of the invention. The summary is notintended to limit the scope of the invention, which is defined solely bythe claims attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The technology disclosed herein, in accordance with one or more variousembodiments, is described in detail with reference to the includedfigures. The drawings are provided for purposes of illustration only andmerely depict typical or example embodiments of the disclosedtechnology. These drawings are provided to facilitate the reader'sunderstanding of the disclosed technology and shall not be consideredlimiting of the breadth, scope, or applicability thereof. It should benoted that for clarity and ease of illustration these drawings are notnecessarily made to scale.

FIG. 1 is a table listing SnAgCuSbBi-system embodiments of a solderalloy in accordance with the disclosure (Alloy Nos. 1-5) and acomparative industry standard high-Pb solder alloy (Alloy No. 6).

FIG. 2 is a table listing embodiments of a solder alloy in accordancewith the disclosure (Alloy Nos. 7-21), and a comparative Pb-freecommercial alloy Sn3.8Ag0.7Cu3.0BiL4Sb0.15Ni (Alloy No. 22-Innolot).

FIG. 3 illustrates the average solder joint shear strengths after 250,500, 1000, and 1500 thermal shock cycles for solder joints comprisingtested Alloy Nos. 1-6 of FIG. 1.

FIG. 4 illustrates the average crack lengths in solder joints measuredafter thermal shock cycles of 500, 1000, and 1500 for solder jointscomprising tested Alloy Nos. 1-6 of FIG. 1.

FIG. 5 shows a set of optical micrographs at the ends of cross-sectionalsolder joints for solder joints comprising tested Alloy Nos. 1-6 afterTS testing of 1000 cycles.

FIG. 6 shows optical micrographs including a close-up view of thecross-sections for a high-PB solder joint after TS testing of 1000cycles.

FIG. 7 shows a set of cross-sectional micrographs of solder joints forAlloy No. 3 and Alloy No. 6 of FIG. 1 after thermal shock testing of 250cycles.

FIG. 8 shows the experimental results of yield strength, ultimatetensile strength, and ductility of as-cast solder alloys listed in FIG.2.

FIG. 9 shows the experimental results of yield strength, ultimatetensile strength, and ductility of the solder alloys listed in FIG. 2after a thermal aging treatment at 200° C. for 1000 hours.

FIG. 10 shows the shear strength variations after 840 and 1585 cycles,respectively, of −55° C./200° C. temperature cycling tests (TCT) for Sidie-attach solder joints on a Cu substrate made from selected solderalloys of FIG. 2. Note that for the Sb3.5 and Sb5.5 alloys, shearstrengths were tested after 860 and 1607 cycles of TCT, respectively.

FIG. 11 shows the shear strength variations after 602 and 1838 cycles of−55° C./200° C. TCT for Si die-attach solder joints on Ni substrate madefrom the same alloys as in FIG. 10.

FIG. 12 shows the shear strength results after 1360 and 2760 cycles,respectively, of −40° C./175° C. TCT for Si die-attach solder joints onCu substrate made from solder alloys in FIG. 2 as well as a comparativehigh-Pb standard alloy.

FIG. 13 shows the shear strength results for the as-reflowed condition,after 1360 and 2760 cycles, respectively, of −40° C./175° C. TCT forInvar die-attach solder joints on Cu substrate made from solder alloysin FIG. 2.

FIG. 14 shows the shear strength results for the as-reflowed condition,after 1360 and 2760 cycles, respectively, of −40° C./175° C. TCT forInvar die-attach solder joints on Ni substrate made from the same alloysas in FIG. 13.

FIG. 15 shows the variations of solidus and liquidus temperatures withIn contents in Sn(3.2-3.8)Ag(0.7-0.9)Cu(3.0-4.0)Sbxln alloys accordingto the present disclosure.

FIG. 16 shows a Sn—Sb binary phase diagram.

The figures are not intended to be exhaustive or to limit the inventionto the precise form disclosed. It should be understood that theinvention can be practiced with modification and alteration, and thatthe disclosed technology be limited only by the claims and theequivalents thereof.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In accordance with various embodiments of the disclosed technology, aSnAgCuSb-based Pb-free solder alloy and solder joints comprising thesolder alloy are disclosed. The disclosed solder alloy is particularlysuitable for, but not limited to, producing solder joints, in the formof solder preforms, solder balls, solder powder, or solder paste (amixture of solder powder and flux), for harsh environment electronicsapplications that require high reliability at higher service oroperation temperatures such as 150° C. or higher.

In various embodiments, the solder alloy comprises 2.5-4.5 wt. % of Ag,0.6-2.0 wt. % of Cu, 2.5-9.0 wt. % of Sb, and the remainder of Sn. Infurther embodiments, the solder alloy may additionally include at leastone of the following additives selected from (a) 0.1-2.5 wt. % of Bi,(b) 0.1-4.5 wt. % of In, and (c) 0.001-0.2 wt. % of Ni, or Co, or both.

In a first set of embodiments, the solder alloy is a SnAgCuSbBi-systemalloy comprising 2.5-4.5 wt. % of Ag, 0.6-2.0 wt. % of Cu, 2.5-9.0 wt. %of Sb, 0.1-2.5 wt. % of Bi, and a remainder of Sn. In particularimplementations of these embodiments, the solder alloy consistsessentially of 3.0-4.0 wt. % of Ag, 0.6-1.2 wt. % of Cu, 5.0-6.0 wt. %of Sb, about 0.3 wt. % of Bi, and the remainder of Sn. For example, thesolder alloy may consist essentially of about 3.8 wt. % of Ag, about 1.0wt. % of Cu, about 6.0 wt. % of Sb, about 0.3 wt. % of Bi, and theremainder of Sn.

In a second set of embodiments, the solder alloy is a SnAgCuSb-systemalloy consisting essentially of 3.0-4.0 wt. % of Ag, 0.6-1.2 wt. % ofCu, 3.0-9.0 wt. % of Sb, and the remainder of Sn. In particularimplementations of these embodiments, the Sb content may be 5.0-6.0 wt.%.

In a third set of embodiments, the solder alloy is aSnAgCuSbIn(Bi)-system alloy comprising: 2.5-4.5 wt. % of Ag, 0.6-2.0 wt.% of Cu, 2.5-9.0 wt. % of Sb, 0.1-4.5 wt. % of In, and the remainder ofSn. In one set of implementations of these embodiments, the solder alloyconsists essentially of 3.0-4.0 wt. % of Ag, 0.6-1.2 wt. % of Cu,3.0-5.0 wt. % of Sb, 1.0-4.0 wt. % of In, about 0.5 wt. % of Bi, and theremainder of Sn. In another set of implementations of these embodiments,the solder alloy consists essentially of 3.0-4.0 wt. % of Ag, 0.6-1.2wt. % of Cu, 5.0-6.0 wt. % of Sb, about 0.5 wt. % of In, and theremainder of Sn.

As illustrated by the experimental results, summarized below, solderjoints made of embodiments of the Pb-free solder alloys disclosed hereinhave a greater thermal fatigue resistance at thermal cycling and thermalshock testing compared to those made of the industry standard high-Pbsolder alloy (Pb5Sn2.5Ag). Additionally, solder joints made ofembodiments of the Pb-free solder alloys disclosed herein substantiallyoutperformed a standard Pb-free commercial alloySn3.8Ag0.7Cu3.0BiL4Sb0.15Ni (Innolot) in thermal fatigue resistance, inshear strength tests under a variety of conditions especially afterthermal cycling tests.

EXAMPLES

The chemical compositions of various embodiments of the disclosedSnAgCuSb-based Pb-free solder alloy (alloy nos. 1-5 and 7-21), anindustry standard high-Pb solder alloy (alloy no. 6), and a Pb-freecommercial alloy Sn3.8Ag0.7Cu3.0Bi1.4Sb0.15Ni (Innolok, aHoy no. 22)were measured with Inductively Coupled Plasma (ICP) analysis, as shownin FIGS. 1 and 2, which list chemical compositions by wt %. The meltingbehavior of the solder alloys was analyzed using Differential ScanningCalorimetry (DSC) with a heating and cooling rate of 10° C./min. DSCtests were performed in a TA Q2000 differential scanning calorimeter,scanning from room temperature to 350° C. For each alloy, the sample wasfirst scanned from ambient temperature up to 350° C., followed bycooling down to 20° C., then scanned again up to 350° C. The secondheating thermograph was used to represent the melting behavior ofalloys. The solidus and liquidus temperatures of solder alloys obtainedfrom the DSC analyses are listed in the tables in FIGS. 1 and 2.

SnAgCuSbBi-system Alloy Examples

Thermal fatigue resistance of solder joints comprising embodiments ofSnAgCuSbBi-system solder alloys, shown in FIG. 1, was evaluated usingthermal shock testing. The thermal shock testing was conducted using thefollowing procedure. Two types of Si die metallized with Ti/Ni/Ag films,with dimensions of 8×8×0.25 mm and 3×3×0.7 mm, respectively, weresoldered to a lead frame substrate of pure Ni coated Cu to formdie-attach solder joints. Solder preforms were used, with dimensions of8×8×0.15 mm and 3×3×0.15 mm, respectively. Soldering was conducted byassembling the set of Si die/solder preform/substrate with a jig tofacilitate the positioning and coplanarity of the assembly, followed byheating in a reflow oven with a peak temperature of 246° C. and timeabove 220° C. of 61 seconds for the Pb-free experimental solder alloys,or with a peak temperature of 335° C. and TAL (time above liquidus) of61 seconds for the standard high-Pb solder alloy (Pb5Sn2.5Ag).

The resulting die-attach solder joints were placed into an air-to-airthermal shock tester, where two separate chambers were set to oppositetemperature extremes, respectively, and a mechanism moved the testedsamples between the two chambers and maintained at each temperatureextreme for a specific time (dwell time). Thermal shock tests werecarried out in the present experiments under −40° C./150° C. with adwell time of 20 minutes for a maximum of 1500 cycles. At cycle numbersof 250, 500, 1000, and 1500, a set of samples were taken out from eachof the 3×3 mm and 8×8 mm type die-attach solder joint samples forvarious testing and measurement purposes. The 3×3 mm type die-attachjoints were shear tested using a Condor 250 XYZTEC die shear tester at ashear speed of 6 mm per minute, and the remaining shear strengths aftervarious thermal shock cycles for each alloy were measured in MPa. The8×8 mm type die-attach joints were used for cracking detection by C-SAMimaging analyses and for crack length measurements by cross-sectioningand microscopy observations.

FIG. 3 illustrates the average solder joint shear strengths after 250,500, 1000, and 1500 thermal shock cycles for solder joints comprisingtested alloys No. 1-6 of FIG. 1. The higher the remaining strength of asolder joint after thermal shock tests, the more reliable it is. Asshown, the shear strength of solder joints decreases with increasingthermal shock cycles due to increased damage in the solder joints causedby thermal shock cycling tests. The decrease rate of the shear strengthsis generally reduced with the increasing thermal shock cycles.

As illustrated, under all test conditions, solder joints includingSnAgCuSbBi embodiments of the disclosed solder alloy (Alloy Nos. 1-5)exhibited higher shear strengths than a solder joint including theindustry standard high-Pb solder alloy (Alloy No. 6). Alloy No. 3 showedshear strengths that doubled those of the high-Pb solder joints underall test conditions.

FIG. 4 illustrates the average crack lengths in solder joints measuredafter thermal shock cycles of 500, 1000, and 1500 for solder jointscomprising tested alloys No. 1-6 of FIG. 1. The average crack lengthincreases with the increase in number of cycles. The slower the crackpropagates in a solder joint, the more resistant it is to thermalfatigue failure. Solder joints including SnAgCuSbBi-system embodimentsof the disclosed solder alloy (Alloy Nos. 1-5) had substantially shorteraverage crack lengths than a solder joint including the industrystandard high-Pb solder alloy (Alloy No. 6).

FIG. 5 shows a set of optical micrographs at the ends of cross-sectionalsolder joints for solder joints comprising tested alloys No. 1-6 afterTS testing of 1000 cycles. The cross-section observations of cracksillustrate the longer crack length of solder joints formed using thehigh-Pb solder alloy (Alloy No. 6). For Alloy No. 6, cracks are observedto propagate from the edge through the whole view field of each picture(as marked by the white arrows). These cracks in the high-Pb solderjoints extended well into the inside of the joints away from both ends,as shown in FIG. 6.

FIG. 7 shows a set of cross-sectional micrographs of solder joints forAlloy No. 3 and Alloy No. 6 after thermal shock testing of 250 cycles.As demonstrated by FIG. 7, for the Pb-free alloys the thermal fatiguecracks generally initiated after 250 cycles of TS testing, whereas forthe high-Pb alloy the cracks initiated before 250 cycles.

SnAgCuSb and SnAgCuSbIn(Bi) System Alloy Examples

Following the good reliability performance of the designed solder alloysin the SnAgCuSbBi system (FIG. 1), alloys in the SnAgCuSb andSnAgCuSbIn(Bi) systems (Alloys No. 11-21), as shown in FIG. 2, weretested.

Tensile tests were conducted in accordance with standard ASTM testingprocedure to evaluate the mechanical properties of the solder alloys.Round test specimens with a diameter of ¼″ and gauge length of 1″ and atesting speed of 0.05 inch per minute were used in the tensile tests.FIG. 8 shows the results of yield strength, ultimate tensile strength,and ductility of as-cast solder alloys listed in FIG. 2. In FIG. 2,alloys No. 7-21 are solder alloys according to the present disclosure,and alloy No. 22 is a commercial high-reliability Pb-free alloy used asa comparative alloy. FIG. 9 shows the results of yield strength,ultimate tensile strength, and ductility of the solder alloys listed inFIG. 2 after a thermal aging treatment at 200° C. for 1000 hours.

Solder pastes were made by mixing the Type 4 (38-20 microns in particlesizes) solder alloy powder (with a metal load of 88.25%) with a no-cleanflux following a standard paste making procedure. Die-attach solderjoints, assembled with a solder paste on either a pure Cu or a pure Nisubstrate, were used for evaluation in the subsequent reliability tests.The Cu substrates were cleaned with a 10% HBF₄ acid solution, thenrinsed with DI water. The pure Ni substrates were cleaned with a nitricacid pickling solution (14% HNO₃ in water), then rinsed with DI water.First, Si dies metallized with Ti/Ni/Ag films (75 nm Ti/300 nm Ni/75 nmAg), with sizes of 3×3×0.7 mm, were used for the solder joints assembly.The solder paste was printed onto the substrate using a stencil of 8mils (0.2 mm) thickness with 3×3 mm openings, and then the die wasplaced onto the printed solder paste. However, for the Si die-attachsolder joints, the solder joint shear strength cannot be measured underthe as-reflowed conditions because in die shear testing of theas-reflowed joints the fracture occurs completely in the Si die. Toobtain the as-reflowed solder joints shear strength, Invar dies withsizes of 3×3×1.0 mm were used to make the metal die-attach solderjoints. Invar is the 64Fe36Ni alloy, known for its uniquely lowcoefficient of thermal expansion (CTE). Invar has a CTE of about 1.2ppm/° C., about half the value compared to a CTE of about 2.6 ppm/° C.for Si. Thus, the Invar die-attach solder joints have a much higher CTEmismatch, and are expected to have a shorter lifetime than the Sidie-attach joints in thermal cycling tests. In the presentinvestigation, the bare Invar dies without coating were cleaned in thesame way as for pure Ni substrate before soldering.

Soldering was conducted in a reflow oven with a peak temperature of243-248° C. and time above liquidus (TAL) of 50-60 seconds for thePb-free solder alloys, or with a peak temperature of 335° C. and TAL ofabout 60 seconds for the standard high-Pb solder alloy (Pb5Sn2.5Ag).

The thermal fatigue resistance of solder joints was evaluated usingaccelerated temperature cycling tests (TCT). The temperature cyclingtests were carried out in the present experiments under a profile of−55° C./200° C. with a dwell time of 5 minutes at each temperatureextreme (about 40 minutes per cycle), as well as under a profile of −40°C./175° C. with a dwell time of 5 minutes at each temperature extreme(about 33 minutes per cycle). At different cycle numbers, a set ofsamples were taken out for die shear testing. The die-attach solderjoints were shear tested using a Condor 250 XYZTEC die shear tester at ashear speed of 6 mm per minute, and the remaining shear strengths aftervarious temperature cycles for each alloy were measured in MPa. For eachcondition, seven solder joints were shear tested.

The test results for the examples of solder alloys according to thepresent disclosure and for comparative alloys (the commercial lnnolotalloy and the industry standard high-Pb solder alloy Indalloy151) areshown in FIGS. 10-14.

FIG. 10 shows the shear strength variations after 840 and 1585 cycles,respectively, of −55° C./200° C. temperature cycling tests (TCT) for Sidie-attach solder joints on a Cu substrate made from selected alloys ofFIG. 2. Note that for the Sb3.5 and Sb5.5 alloys, shear strengths weretested after 860 and 1607 cycles of TCT, respectively. The higher theremaining strength of a solder joint after TCT, the more reliable it is.It is demonstrated that the shear strength of solder joints decreaseswith increasing thermal cycles due to increased damage in the solderjoints caused by TCT. As shown in FIG. 10, solder joints made from thenew exemplary solder alloys exhibit higher average remaining shearstrengths than the commercial Innolot solder joint after TCT under thesetest conditions. The Sl6005 alloy with a composition ofSn3.8Ag0.9Cu6.0Sb0.5In more than doubled the performance of thecomparative alloy.

FIG. 11 shows the shear strength variations after 602 and 1838 cycles of−55° C./200° C. TCT for Si die-attach solder joints on Ni substrate madefrom the same alloys as in FIG. 10. Generally, the Si die-attach solderjoints on Ni substrate have a much higher average remaining shearstrength after TCT than their counterparts on Cu substrate. This is dueto the fact that the mismatch of coefficient of thermal expansion (CTE)in the solder joints of Ni substrate (with CTE 13 ppm/° C. for Ni vs. ˜3ppm/° C. for Si) is lower than the CTE mismatch in those of Cu substrate(with CTE 17 ppm/° C. for Cu vs. ˜3 ppm/° C. for Si). As a result, lowerstresses or strains and thus less damage are produced in the solderjoints of Ni substrate than in those of Cu substrate during thetemperature cycling tests. As illustrated in FIG. 11, all solder jointson Ni made from the exemplary solder alloys exhibit higher average shearstrengths than the comparative Innolot solder joint after TCT underthese test conditions. The Sl6005, SB6003 (with a nominal composition ofSn3.8Ag1.0Cu6.0Sb0.3Bi), and Sb5.5 (with a nominal composition ofSn3.8Ag1.0Cu5.5Sb) were the three best performers in these tests.

FIG. 12 shows the shear strength results after 1360 and 2760 cycles,respectively, of −40° C./175° C. TCT for Si die-attach solder joints onCu substrate made from exemplary solder alloys as well as from thecomparative alloys (the Innolot alloy and the Indalloy151 high-Pbstandard alloy). As illustrated in FIG. 12, under these test conditions,all solder joints on Cu made from the exemplary solder alloys exhibithigher average shear strengths after TCT not only than the comparativePb-free Innolot solder joint, but also than the comparative high-PbIndalloy151 solder joint.

FIG. 13 shows the shear strength results for the as-reflowed condition,after 1360 and 2760 cycles, respectively, of −40° C./175° C. TCT forInvar die-attach solder joints on Cu substrate made from exemplarysolder alloys in FIG. 2 as well as from the comparative Innolot alloy.The Indalloy151 high-Pb solder alloy was not included in the evaluationas a comparative alloy because the solder joints made with the bareInvar die were weak due to the poor wetting of the high-Pb solder on theInvar alloy. As mentioned previously, one of the advantages in using theInvar dies for making the metal die-attach solder joints is to be ableto measure the as-reflowed solder joints shear strengths. As illustratedin FIG. 13, the as-reflowed solder joints on Cu substrate made from theexemplary solder alloys have very high average shear strengths, rangingfrom 76 to 94 MPa, as compared to 68 MPa for the comparative Innolotsolder joint. After 1360 and 2760 cycles of −40° C./175° C. TCT, theaverage shear strengths of solder joints decrease significantly, butthose of solder joints made from the exemplary solder alloys are muchhigher than the Innolot solder joints shear strength.

FIG. 14 shows the shear strength results for the as-reflowed condition,after 1360 and 2760 cycles, respectively, of −40° C./175° C. TCT forInvar die-attach solder joints on Ni substrate made from the same alloysas in FIG. 13. Generally, the as-reflowed Invar die-attach solder jointson Ni substrate have a similar average shear strength to theircounterparts on Cu substrate. However, the decrease rates of the shearstrengths are considerably reduced after TCT for the Invar die-attachsolder joints on Ni substrate, as compared to their counterparts on Cusubstrate. As illustrated in FIG. 14, after TCT under these testconditions, all Invar die-attach joints on Ni made from the exemplarysolder alloys exhibit not only higher average shear strengths, but alsomuch reduced decrease rates of shear strengths from the as-reflowedcondition, than the comparative Innolot solder joint.

FIG. 15 shows the variations of solidus and liquidus temperatures withIn contents in Sn(3.2-3.8)Ag(0.7-0.9)Cu(3.0-4.0)Sbxln alloys accordingto the present disclosure. As the In concentration increases, both thesolidus and liquidus temperatures are reduced. Thus, the addition of Into the SnAgCuSb alloys can effectively decrease the melting temperatureof the alloy. As the In concentration increases, the melting temperaturerange (between solidus and liquidus) also becomes wider. For solderingperformance considerations, a narrow melting temperature range less than15° C. is desired, the In addition should be no more than 5 wt %.

FIG. 16 shows a Sn—Sb binary phase diagram. Based on the equilibriumphase diagram, a Sn(Sb) solid solution forms after solidification of aSn—Sb alloy of less than 10.2 wt % Sb. Upon subsequent cooling the SnSbintermetallic phase is precipitated from the supersaturated Sn(Sb) solidsolution (>3 wt % Sb). The β-SnSb phase is a quasicubic NaCI (B1)face-centered cubic (FCC) type. This structure contains one sublatticeof Sb atoms and another sublattice of Sn atoms, with each Sn atom beingsurrounded by six Sb first neighbors and each Sb atom by six Sn firstneighbors. Since its compositions vary within a rather wide range, thistype of intermetallic phases tends to be moderately ductile and thushave a benign effect on joint properties. Thus, the addition of Sb >3 wt% can provide both solid-solution and precipitate strengthening to theSnAgCu base alloy. For a Sn—Sb alloy with Sb content between 6.7 wt %and 10.2 wt %, a primary solidification phase of Sn₃Sb₂ forms initially,and converts into the Sn(Sb) solid solution via a peritectic reaction at250° C., as shown in the Sn—Sb binary phase diagram. However, in anon-equilibrium solidification condition like the soldering process,this Sn₃Sb₂ phase conversion cannot be complete, and the coarse primaryintermetallic phase tends to be brittle. Thus, the Sb content in thealloys according to the present disclosure is preferred to be belowabout 9 wt %.

Benefits of the Compositional Ranges of Alloys of the Present Disclosure

The benefits of the compositional range of alloys disclosed herein aredescribed below. In the Sn—Ag—Cu alloy system, the ternary eutecticcomposition is approximately Sn3.7Ag0.9Cu, with a eutectic temperatureof 217° C. Ag is a major strengthening element in the alloy by formingthe Ag₃Sn intermetallic particles to act as dispersion strengtheningphases. Ag also improves the wettability of solder alloys. Forcomprehensive considerations of alloys melting behavior, wetting,mechanical properties and thermal cycling reliability, the Ag content ispreferred to be in the range of 2.5-4.5 wt %. When Ag is less than 2.5wt %, mechanical properties and thermal cycling reliability performanceof solder joints are not good enough for harsh environment electronicsapplications. When Ag is more than 4.5 wt %, the alloy's liquidustemperature is increased significantly, and soldering performance isadversely affected. In addition, the cost increase with higher Agcontents is not desired. Accordingly, in embodiments the Ag content ispreferably in the range of 3.0-4.0 wt %.

As one of the major elements constituting the SnAgCuSb base alloy, Cuimproves the alloy's mechanical properties by the formation of Cu₆Sn₅intermetallic particles in the solder matrix. It also greatly reducesthe dissolution of Cu substrate metal or Cu pads. Based on observationsof solder joint microstructure, it was found by the inventors that ahigher Cu content in the solder can improve the reliability of solderjoints especially with Ni substrate metal or surface finishes bypromoting and stabilizing a (Cu,Ni)₆Sn₅ intermetallic layer structureand preventing the (Cu,Ni)₆Sn₅/(Cu,Ni)₃Sn₄ dual layer structure fromformation at the solder joint interfaces. Furthermore, a higher Cucontent in the solder can also suppress the occurrences of Ag₃Sn platesin solder joints with high Ag content (3 wt % or higher) by initiatingthe Cu₆Sn₅ primary solidification instead of the Ag₃Sn primarysolidification phase formation. When Cu is less than 0.6 wt %, theabove-mentioned beneficial effects are not expected to be utilized. WhenCu is more than 2.0 wt %, the alloys liquidus temperature becomes toohigh and the melting temperature range becomes too wide for reflowsoldering, which affects the soldering performance adversely (e.g.,increased voiding). In embodiments of the present disclosure, the Cucontent is preferably in the range of 0.6-1.2 wt %.

In the present disclosure, Sb is found to be a key element improving thethermal fatigue resistance of solder joints made of the disclosed alloysin very severe thermal cycling or thermal shock testing conditions usedin the present investigations. When the Sb content is less than 2.5 wt%, Sb is dissolved in the Sn matrix to form a Sn(Sb) solid solution aswell as in the Ag₃Sn phase. As mentioned previously, with the additionof Sb >3 wt % in the solder alloys, the β-SnSb (FIG. 16) intermetallicphase is precipitated from the supersaturated Sn(Sb) solid solution,providing both solid-solution and precipitation strengthening to theSnAgCu alloy. Due to the characteristics and benign effects of theβ-SnSb intermetallic precipitation strengthening mechanism, the SnAgCuSballoys according to the present disclosure exhibit superiorcomprehensive mechanical properties (both high strength and highductility), as shown in FIG. 8, as well as greatly improved solder jointreliability performance. However, the addition of Sb increases both thesolidus and liquidus temperatures of the alloy. Furthermore, based onprevious analyses, in order to avoid the complication of a coarse andbrittle primary solidification phase of Sn₃Sb₂, the Sb content accordingto the present disclosure should be below about 9 wt %. Sb content ismore preferably in the range of 3.0-8.0 wt %. Based on the reliabilitytest results in the present investigations, an optimum Sb content isabout 5-6 wt % for SnAgCuSb alloys.

As additives to the SnAgCuSb alloys, both Bi and In can decrease thesolidus and liquidus temperatures of the alloy. Bi also reduces thesurface tension of liquid solders, and thus improves the alloyswettability. Unlike Sb, when Bi is more than 2.5 wt %, the Bi additionincreases the alloys strengths, but reduces its ductility significantly,making solder joints brittle with decreased thermal fatigue resistance.In embodiments of the present disclosure, a Bi addition of 1.5 wt % orbelow is preferred for harsh environment electronics applications.

In addition to the beneficial effects of reducing the solidus andliquidus temperatures of the alloy, when In is added to the SnAgCuSballoys in less than 4.5 wt %, In is mostly dissolved in the β-Sn matrixto provide a solid-solution strengthening effect. Thus, the alloysmechanical properties and solder joints thermal cycling reliabilityperformance are further improved. Based on microstructure observationsof solder joints subjected to severe temperature cycling tests, it wasfound by the inventors that In additions to the SnAgCuSb alloys can alsostrengthen grain boundaries and suppress the grain boundary damages athigh temperatures, and delay the recrystallization process of solderjoints during temperature cycling testing. As discussed previously, whenIn content is 5 wt % or higher, the alloys melting temperature range islarger than 15° C. In is also an alloying element prone to oxidation,especially in the form of fine solder powder for solder pasteapplications. It was found by the inventors that soldering performanceis decreased (e.g., reduced wetting and increased voiding) with alloysof In additions higher than 4.5 wt %. Thus, In addition of 4.5 wt % orbelow is generally preferred in the present disclosure. A preferred Incontent in the alloy also depends on the Sb content. When Sb content ishigher than 5.0 wt %, the In addition is preferred to be less than 3.0wt % to avoid incipient melting phases in the alloy.

In the present disclosure, an amount of 0.001-0.2 wt. % of Ni, or Co, orboth can be added to further improve the alloy's mechanical propertiesand solder joint reliability performance. When the total amount ishigher than 0.2 wt %, the alloy's liquidus temperature is increasedexcessively. In addition, these elements are also prone to oxidation,and thus adversely affect soldering performance when the total additionis more than 0.2 wt %, especially in the form of fine solder powder forsolder paste applications. Thus, the upper limit for these additions is0.2 wt %.

While various embodiments of the disclosed technology have beendescribed above, it should be understood that they have been presentedby way of example only, and not of limitation. Likewise, the variousdiagrams may depict an example architectural or other configuration forthe disclosed technology, which is done to aid in understanding thefeatures and functionality that can be included in the disclosedtechnology. The disclosed technology is not restricted to theillustrated example architectures or configurations, but the desiredfeatures can be implemented using a variety of alternative architecturesand configurations. Indeed, it will be apparent to one of skill in theart how alternative functional, logical or physical partitioning andconfigurations can be implemented to implement the desired features ofthe technology disclosed herein. Also, a multitude of differentconstituent module names other than those depicted herein can be appliedto the various partitions. Additionally, with regard to flow diagrams,operational descriptions and method claims, the order in which the stepsare presented herein shall not mandate that various embodiments beimplemented to perform the recited functionality in the same orderunless the context dictates otherwise.

Although the disclosed technology is described above in terms of variousexemplary embodiments and implementations, it should be understood thatthe various features, aspects and functionality described in one or moreof the individual embodiments are not limited in their applicability tothe particular embodiment with which they are described, but instead canbe applied, alone or in various combinations, to one or more of theother embodiments of the disclosed technology, whether or not suchembodiments are described and whether or not such features are presentedas being a part of a described embodiment. Thus, the breadth and scopeof the technology disclosed herein should not be limited by any of theabove-described exemplary embodiments.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as meaning “including, without limitation” or the like; the term“example” is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; the terms “a” or“an” should be read as meaning “at least one,” “one or more” or thelike; and adjectives such as “conventional,” “traditional,” “normal,”“standard,” “known” and terms of similar meaning should not be construedas limiting the item described to a given time period or to an itemavailable as of a given time, but instead should be read to encompassconventional, traditional, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, wherethis document refers to technologies that would be apparent or known toone of ordinary skill in the art, such technologies encompass thoseapparent or known to the skilled artisan now or at any time in thefuture.

The presence of broadening words and phrases such as “one or more,” “atleast,” “but not limited to” or other like phrases in some instancesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent. The use of theterm “module” does not imply that the components or functionalitydescribed or claimed as part of the module are all conFIG.d in a commonpackage. Indeed, any or all of the various components of a module,whether control logic or other components, can be combined in a singlepackage or separately maintained and can further be distributed inmultiple groupings or packages or across multiple locations.

Additionally, the various embodiments set forth herein are described interms of exemplary block diagrams, flow charts and other illustrations.As will become apparent to one of ordinary skill in the art afterreading this document, the illustrated embodiments and their variousalternatives can be implemented without confinement to the illustratedexamples. For example, block diagrams and their accompanying descriptionshould not be construed as mandating a particular architecture orconfiguration.

What is claimed is:
 1. A solder alloy, consisting of: 2.5-4.5 wt. % ofAg; 0.6-2.0 wt. % of Cu; greater than 5.0 wt. % to 6.0 wt. % of Sb;optionally, 0.001-0.2 wt. % of Ni, or Co, or both; and a remainder ofSn.
 2. The solder alloy of claim 1, consisting of: 3.0-4.0 wt. % of Ag;0.6-1.2 wt. % of Cu; greater than 5.0 wt. % to 6.0 wt. % of Sb;optionally, 0.001-0.2 wt. % of Ni, or Co, or both; and the remainder ofSn.
 3. The solder alloy of claim 2, consisting of: 3.0-4.0 wt. % of Ag;0.6-1.2 wt. % of Cu; greater than 5.0 wt. % to 6.0 wt. % of Sb;0.001-0.2 wt. % of Ni, or Co, or both; and the remainder of Sn.
 4. Thesolder alloy of claim 3, consisting of: 3.0-4.0 wt. % of Ag; 0.6-1.2 wt.% of Cu; greater than 5.0 wt. % to 6.0 wt. % of Sb; 0.001-0.2 wt. % ofNi; and the remainder of Sn.
 5. The solder alloy of claim 2, consistingof: 3.0-4.0 wt. % of Ag; 0.6-1.2 wt. % of Cu; greater than 5.0 wt. % to6.0 wt. % of Sb; and the remainder of Sn.
 6. The solder alloy of claim1, wherein the solder alloy is a solder ball.
 7. The solder alloy ofclaim 1, wherein the solder alloy is a solder preform.
 8. The solderalloy of claim 1, wherein the solder alloy is a solder powder.
 9. Thesolder alloy of claim 2, wherein the solder alloy is a solder ball. 10.The solder alloy of claim 2, wherein the solder alloy is a solderpreform.
 11. The solder alloy of claim 2, wherein the solder alloy is asolder powder.
 12. A solder paste, consisting of: flux; and a solderalloy powder, consisting of: 2.5-4.5 wt. % of Ag; 0.6-2.0 wt. % of Cu;greater than 5.0 wt. % to 6.0 wt. % of Sb; optionally, 0.001-0.2 wt. %of Ni, or Co, or both; and a remainder of Sn.
 13. The solder paste ofclaim 12, the solder alloy powder consisting of: 3.0-4.0 wt. % of Ag;0.6-1.2 wt. % of Cu; greater than 5.0 wt. % to 6.0 wt. % of Sb;optionally, 0.001-0.2 wt. % of Ni, or Co, or both; and the remainder ofSn.
 14. The solder paste of claim 13, the solder alloy powder consistingof: 3.0-4.0 wt. % of Ag; 0.6-1.2 wt. % of Cu; greater than 5.0 wt. % to6.0 wt. % of Sb; 0.001-0.2 wt. % of Ni, or Co, or both; and theremainder of Sn.
 15. The solder paste of claim 14, the solder alloypowder consisting of: 3.0-4.0 wt. % of Ag; 0.6-1.2 wt. % of Cu; greaterthan 5.0 wt. % to 6.0 wt. % of Sb; 0.001-0.2 wt. % of Ni; and theremainder of Sn.
 16. The solder paste of claim 13, the solder alloypowder consisting of: 3.0-4.0 wt. % of Ag; 0.6-1.2 wt. % of Cu; greaterthan 5.0 wt. % to 6.0 wt. % of Sb; and the remainder of Sn.
 17. A solderjoint formed by a process, the process comprising: applying a solderalloy between a substrate and a device to form an assembly; and reflowsoldering the assembly to form the solder joint; wherein the solderalloy consists of: 2.5-4.5 wt. % of Ag; 0.6-2.0 wt. % of Cu; greaterthan 5.0 wt. % to 6.0 wt. % of Sb; optionally, 0.001-0.2 wt. % of Ni, orCo, or both; and a remainder of Sn.
 18. The solder joint of claim 17,wherein the solder alloy consists of: 3.0-4.0 wt. % of Ag; 0.6-1.2 wt. %of Cu; greater than 5.0 wt. % to 6.0 wt. % of Sb; optionally, 0.001-0.2wt. % of Ni, or Co, or both; and the remainder of Sn.
 19. The solderjoint of claim 18, wherein the solder alloy consists of: 3.0-4.0 wt. %of Ag; 0.6-1.2 wt. % of Cu; greater than 5.0 wt. % to 6.0 wt. % of Sb;and the remainder of Sn.
 20. The solder joint of claim 18, wherein thesolder alloy is a solder alloy of a solder paste consisting of flux anda powder of the solder alloy, wherein applying the solder alloycomprises applying the solder paste.